CHAPTER IX. 



THE MOVEMENTS AND DEFORMATIONS OF THE EARTH'S BODY 



(DIASTROPHISM). 



The body of the earth is subject to an infinite variety of movements, 

 ranging from the almost inconceivably rapid to the almost impercept- 

 ibly slow, and from the almost immeasurably minute to the enormously 

 massive; but, for practical treatment, they fall mainly into two couplets: 

 (1) the minute and rapid, and (2) the slow and massive. Sudden move- 

 ments of local masses, giving rise to intense vibrations, are put in the first 

 class. There are innumerable minute and slow movements, but un- 

 less they rise to appreciable magnitude by long continuance, they are 

 neglected. 



MINUTE AND RAPID MOVEMENTS. 



The crust of the earth is in a state of perpetual tremor. For the 

 most part, these tremors are too minute to be sensible, but are revealed 

 by delicate instrumental devices. Some of them are but the declining 

 stages of sensible vibrations, but others are minute from their inception. 

 Many of them spring from the ordinary incidents of the surface, and 

 claim attention chiefly as obstacles to the study of more significant 

 oscillations. Winds, waves, waterfalls, the tread of animals, the rumble 

 of traffic, the blasts of mines, the changes of temperature, the variations 

 in atmospheric pressure, the weighting of rainfall and the lightening of 

 evaporation, the rupture of rock or ice or frozen earth, and many other 

 processes, make their contributions to local and minute movements. 

 For the greater part, these vibrations are superficial in origin, and are 

 soon damped beyond recognition by dispersal and by the inelastic 

 and discontinuous nature of the looser material of the surface. When 

 a temporary rigid crust is formed by freezing, as in winter, these sur- 

 face vibrations are transmitted with much less loss, and the distances 

 at which the rumble of winter traffic is heard, is a good illustration of 

 the function of continuity and solidity in the conveyance of vibrations. 



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